1. Field of the Invention PA1 2. Description of the Related Art
The invention relates to a method of determining the conditions of a reflow furnace in which a semiconductor element is mounted on a substrate and, more particularly, to a method of preventing the displacing of melting of solder bumps.
FIG. 11 shows an outline of the reflow furnace for heating solder so as to mount the semiconductor element on the substrate. Terminals of packaging parts referred to as BGA or CSP, or of the semiconductor element 1 comprising bare chips are, as shown, formed with solder bumps 2, as shown in FIG. 12(a), and positioned and mounted onto a substrate 3. The substrate 3 is placed in a mounting jig 4 made by carbon and conveyed onto a conveyor belt. For securing connection with a the solder bumps 2, the semiconductor element 1 is loaded with a weight 5 of several milligrams for each bump 2 to be mashed when the solder bumps 2 are melted. Thus, the substrate 3 and the semiconductor element 1 are moved by the conveyor belt into the reflow furnace 6, and heated by a heat of a gas within the reflow furnace 6. The heat thereby is transferred to the solder bumps 2, the solder bumps 2 are melted, and the semiconductor element 1 and the substrate 3 are connected. The temperature in the reflow furnace 6 is set to different temperatures in, e.g., six zones, as shown in the figure. The temperature is set to rise from the entrance side of the reflow furnace toward the inside, and is then set to drop at the exit portion. The zone at the entrance side, called the "preheating zone", is constructed such that even if mounting parts are different, a solder for mounting a plurality of mounting parts having the semiconductor element 1 in a lump has a uniform temperature. The following zone is called the "reflow zone", and is set at a temperature at which the solder can be melted. Assuming that these six zones are R1, R2, R3, R4, R5, and R6, respectively, the temperature relationship thereof is set as R1&lt;R6&lt;R2&lt;R3=R5&lt;R4. If the solder bumps have a high concentration of Pb, since the melting point is around 310.degree. C., R1 is set around 300.degree. C., which is a temperature just under the melting temperature, R2 is set around 400.degree. C., and R6 is set around 350.degree. C., which is a temperature after the melting.
Although the temperature is thus set such that the solder is melted appropriately, there are some cases of which shortage phenomena occur at the connecting portion of the solder bumps. Taking statistics of such phenomena for each of the solder bumps of one semiconductor element, it is found that more shortages occurred around the solder bumps at the forward side of the advancing direction within a reflow furnace. That is, as shown in FIGS. 12(a)-12(b), a semiconductor element 1 mounted via solder bumps 2 on a substrate 3 housed in a mounting jig 4 is moved to the right as shown in FIG. 12(a). Then, as shown in FIG. 12(b), the semiconductor element 1 is mounted at a position lower than a regular mounting position 1. Further, at this time the mashed amount of the solder bumps is greater at the right side of the figure, which is the forward side of the advancing direction.
To analyze the phenomena in greater detail, the temperature history of the interior of the furnace was acquired and the temperature was analyzed using ABAQUS (product name), which is a general purpose analysis program of a finite element method of Hibbitt, Karlsson & Sorensen Inc., U.S. It was found that when the analysis was carried out, the temperature histories were different, in terms of time, between the solder bumps in the forward side and the solder bumps in the backward side of the advancing direction in the reflow furnace among the solder bumps to be provided on the same semiconductor element 1.
It is assumed that when the solder bumps reach the melting temperature, the mashed amount of the solder bump increases, and the sinking amount of the semiconductor element becomes larger as time progresses. If this condition occurs excessively, it is assumed that the solder bumps spread laterally, the adjacent solder bumps are fused together, and then the shorting phenomena occur.
The invention is directed toward removing the failure of connections caused by which the meshed amounts of the plurality of solder bumps for connecting the semiconductor element to the substrate are different within a single semiconductor element.